Diamond heat spreading and cooling technique for integrated circuits

ABSTRACT

A semiconductor chip is shown containing an integral heat spreading layer that more effectively transmits heat from the die to ambient as a result of spreading the heat out on the die over a larger cross sectional area. Local hot spots are minimized which allows the semiconductor chip to operate at a higher frequency for a given upper threshold temperature. Also shown is a method of manufacturing such a semiconductor chip, and the associated method of cooling a semiconductor chip.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to the field ofelectronic devices and, in particular, the present invention relates tothermal management of electronic devices.

BACKGROUND

[0002] Integrated circuits (IC's) are typically assembled into packagesby physically and electrically coupling them to a substrate made oforganic or ceramic material. One or more IC packages can be physicallyand electrically coupled to a printed circuit board (PCB) to form an“electronic assembly”. The “electronic assembly” can be part of an“electronic system”. An “electronic system” is broadly defined herein asany product comprising an “electronic assembly”. Examples of electronicsystems include computers (e.g., desktop, laptop, hand-held, server,etc.), wireless communications devices (e.g., cellular phones, cordlessphones, pagers, etc.), computer-related peripherals (e.g., printers,scanners, monitors, etc.), entertainment devices (e.g., televisions,radios, stereos, tape and compact disc players, video cassetterecorders, MP3 (Motion Picture Experts Group, Audio Layer 3) players,etc.), and the like.

[0003] In the field of electronic systems there is an incessantcompetitive pressure among manufacturers to drive the performance oftheir equipment up while driving down production costs. This isparticularly true regarding forming electronic devices such astransistors in IC's, where each new generation of IC must provideincreased performance, particularly in terms of an increased number ofdevices and higher clock frequencies, while generally being smaller ormore compact in size. As the density and clock frequency of IC'sincrease, they accordingly generate a greater amount of heat. However,the performance and reliability of IC's are known to diminish as thetemperature to which they are subjected increases, so it becomesincreasingly important to adequately dissipate heat from ICenvironments.

[0004] An IC is fabricated on a substrate that may comprise a number ofmetal layers selectively patterned to provide metal interconnect lines(referred to herein as “traces”), and one or more electronic devicesattached in or on one or more surfaces of the substrate. The electronicdevice or devices are functionally connected to other elements of anelectronic system through a hierarchy of electrically conductive pathsthat include the substrate traces. The substrate traces typically carrysignals that are transmitted between the electronic devices, such astransistors, of the IC. Some IC's have a relatively large number ofinput/output (I/O) terminals (also called “lands”), as well as a largenumber of power and ground terminals or lands.

[0005] As the internal circuitry of IC's, such as processors, operatesat higher and higher clock frequencies, and as IC's operate at higherand higher power levels, the amount of heat generated by such IC's canincrease their operating temperature to unacceptable levels. Thermalmanagement of IC's refers to the ability to keep temperature-sensitiveelements in an IC within a prescribed operating temperature. Thermalmanagement has evolved to address the increased temperatures createdwithin such electronic devices as a result of increased processingspeed/power of the electronic devices.

[0006] With the advent of high performance processors, electronicdevices have required more innovative thermal management. For example,in the last several years processing speeds of computer systems haveclimbed from 25 MHZ to over 1000 MHZ. Each of these increases inprocessing speed and power generally carry with it a “cost” of increasedheat that must be dissipated. Corresponding improvements in thermalmanagement such as improved heat sinks or heat pipes have accompaniedsuch technological improvements. Further improvements in thermalmanagement are needed to keep pace with ever increasing processor speedsand the desire to reduce manufacturing costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of an electronic system incorporating atleast one electronic assembly with a high capacity thermal interface inaccordance with one embodiment of the invention.

[0008]FIG. 2 illustrates a cross-sectional representation of a prior artIC package.

[0009]FIG. 3a is a cross-sectional representation of a first embodimentof an IC package according to the invention.

[0010]FIG. 3b is a cross-sectional representation of a second embodimentof an IC package according to the invention.

[0011]FIG. 3c is a cross-sectional representation of a third embodimentof an IC package according to the invention.

[0012]FIG. 4 is a magnified view of the first embodiment in FIG. 3a.

[0013]FIG. 5 is a perspective view of a die according to the invention

[0014]FIG. 6 is a diagram of a diamond crystal structure unit cell.

[0015]FIG. 7a is an experimental data thermal map of a prior artsemiconductor chip.

[0016]FIG. 7b is an experimental data thermal map of a semiconductorchip according to the invention.

DETAILED DESCRIPTION

[0017] In the following detailed description of the invention referenceis made to the accompanying drawings which form a part hereof, and inwhich are shown, by way of illustration, specific embodiments in whichthe invention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilized,and structural, logical, and electrical changes may be made, withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

[0018] The term “active side” as used in this description is defined asthe conventional horizontal, large plane or surface of a chip or diewhere electrical devices have typically been fabricated, regardless ofthe orientation of the chip or die. The term “back side” as used in thisdescription is defined as a conventional horizontal, large plane orsurface of a chip or die that does not contain any active devices onit's surface. The term “vertical” refers to a direction perpendicular tothe horizontal as defined above. Prepositions, such as “on”, “higher”,“lower”, “above” and “below” are defined with respect to theconventional plane or surface being on the active side of the chip ordie, regardless of the orientation of the chip or die.

[0019] The present invention provides a solution to thermal dissipationproblems that are associated with prior art integrated circuits thathave high circuit density and that operate at high clock speeds and highpower levels, by employing a high capacity thermal material as a heatspreading layer that is integral with the integrated circuit die.Various embodiments are illustrated and described herein.

[0020]FIG. 1 is a block diagram of an electronic system 1 incorporatingat least one electronic assembly 4 with a high capacity heat spreadinglayer in accordance with one embodiment of the invention. Electronicsystem 1 is merely one example of an electronic system in which thepresent invention can be used. In this example, electronic system 1comprises a data processing system that includes a system bus 2 tocouple the various components of the system. System bus 2 providescommunications links among the various components of the electronicsystem 1 and can be implemented as a single bus, as a combination ofbusses, or in any other suitable manner.

[0021] Electronic assembly 4 is coupled to system bus 2. Electronicassembly 4 can include any circuit or combination of circuits. In oneembodiment, electronic assembly 4 includes a processor 6 which can be ofany type. As used herein, “processor” means any type of computationalcircuit, such as but not limited to a microprocessor, a microcontroller,a complex instruction set computing (CISC) microprocessor, a reducedinstruction set computing (RISC) microprocessor, a very long instructionword (VLIW) microprocessor, a graphics processor, a digital signalprocessor (DSP), or any other type of processor or processing circuit.

[0022] Other types of circuits that can be included in electronicassembly 4 are a custom circuit, an application-specific integratedcircuit (ASIC), or the like, such as, for example, one or more circuits(such as a communications circuit 7) for use in wireless devices likecellular telephones, pagers, portable computers, two-way radios, andsimilar electronic systems. The IC can perform any other type offunction.

[0023] Electronic system 1 can also include an external memory 10, whichin turn can include one or more memory elements suitable to theparticular application, such as a main memory 12 in the form of randomaccess memory (RAM), one or more hard drives 14, and/or one or moredrives that handle removable media 16 such as floppy diskettes, compactdisks (CDS), digital video disk (DVD), and the like.

[0024] Electronic system 1 can also include a display device 8, one ormore speakers 9, and a keyboard and/or controller 20, which can includea mouse, trackball, game controller, voice-recognition device, or anyother device that permits a system user to input information into andreceive information from the electronic system 1.

[0025]FIG. 2 illustrates a cross-sectional representation of a commonconfiguration IC package 30. IC package 30 represents a typicalstructure that includes an IC die 40 mounted in “flip-chip” orientationwith its active side facing downward to couple with lands 52 on theupper surface of a board 50 through solder balls or bumps 42. Board 50can be a one-layer board or a multi-layer board, and it can includeadditional lands 54 on its opposite surface for mating with additionalpackaging structure (not shown).

[0026] Die 40 generates its heat from internal structure, includingwiring traces, that is located near its active side; however, asignificant portion of the heat is dissipated through its back side.Heat that is concentrated within die 40 is dissipated to a large surfacethat is in contact with die 40 in the form of an external heat spreader60 that is typically formed of metal such as copper or aluminum. Toimprove the thermal conductivity between die 40 and the external heatspreader 60, a thermal interface material 70 is often provided betweendie 40 and external heat spreader 60. The thermal interface material 70may include a thermal gel or grease containing metal particles, or inanother embodiment, it may include solder.

[0027] To further dissipate heat from external heat spreader 60, a heatsink 80 optionally having heat fins 82 is often coupled to external heatspreader 60. Heat sink 80 dissipates heat into the ambient environment.

[0028] An increase in the transistor junction temperature T_(j) of anelectronic device on the IC can adversely affect the operating lives ofthe device. Transistor junction temperature is a function of threefactors: junction-to-ambient thermal resistance, power dissipation, andambient temperature. T_(j) can be expressed by Equation 1:

T _(j)=(θ_(ja) ×P _(d))+T _(a)  (Equation 1)

[0029] wherein T_(j)=transistor junction temperature (in degrees C.);

[0030] θ_(ja)=the junction-to-ambient thermal resistance (in degreesC./watt);

[0031] P_(d)=power dissipation at T_(j) (in watts); and

[0032] T_(a)=ambient temperature (in degrees C.).

[0033] The junction-to-ambient thermal resistance θ_(ja) can berepresented by Equation 2:

θ_(ja)=θ_(jc)+θ_(cs)+θ_(sa)  (Equation 2)

[0034] wherein θ_(jc)=the junction-to-package case thermal resistance(in degrees C./watt);

[0035] θ_(cs)=the case-to-sink thermal resistance (in degrees C./watt);and

[0036] θ_(sa)=the sink-to-ambient thermal resistance (in degreesC./watt);

[0037] In the foregoing definitions, the pertinent location of the caseis the top center of the IC package. The pertinent location of the sinkcan be the geometric center of the heat sink.

[0038] The IC package 30 of FIG. 2 limits the capability of meeting thethermal-dissipating requirements of today's high performance electronicassemblies, as expressed in terms of the junction-to-ambient thermalresistance θ_(ja).

[0039] The present invention reduces the transistor junction temperatureT_(j) by reducing the effective junction-to-package case thermalresistance θ_(jc). As shown in FIG. 7a, processor assemblies have a verynon-uniform power map or heat flux variation across the surfaces of thedie. It is the temperature of the highest flux area(s) that typicallymust be maintained at or below a specified value. While the silicon dieprovides some lateral heat spreading, it is insufficient to appreciablyreduce the peak temperature(s).

[0040] If existing thermal dissipation structure is incapable ofdissipating sufficient heat to maintain the die peak temperature below aspecified value, the performance of the electronic assembly must bethrottled back by reverting to a temperature-dependent processor powercontrol process. Otherwise, the electronic assembly could malfunction orexperience a catastrophic failure. Thus, with increased heat dissipationrequirements for electronic assemblies, it has become necessary to looktowards new techniques and materials for thermal management.

[0041]FIGS. 3a-3 c show the addition of a heat spreading layer 100formed on or within the die 40 according to the invention. The heatspreading layer is formed as an integral part of the die 40. Integrallyforming the heat spreading layer entails forming the layer during thechip fabrication process. The layer effectively becomes a part of thedie 40. Using this method, a thin film such as the 0.05 mm heatspreading layer 100 can be added to the die, and the die providessupport for the film. If a film were added separately, instead ofintegral formation, it would be extremely fragile and impractical tomanufacture separately. Additionally, integrally forming the heatspreading layer 100 has the advantage of a high surface contact area.Any surface features of the die are easily covered during the formingprocess and covering all surface areas of such features increases thespeed and effectiveness of heat conduction into the heat spreadinglayer. Locating the heat spreading layer within the die, for example,beneath electrical devices such as transistors is also easilyaccomplished by integrally forming the heat spreading layer.

[0042] In FIG. 3a, the heat spreading layer 100 is formed on an activeside of a semiconductor portion 110 of the die 40. In FIG. 3b, the heatspreading layer is formed on the back side of the semiconductor portion110. In FIG. 3c, the heat spreading layer 100 is formed as anintermediate step in the formation of the die 40. In FIG. 3c, the heatspreading layer 100 divides the die 40 into a first semiconducting layer112, and a second semiconducting layer 114. While the exact location ofthe heat spreading layer 100 within the die 40 is not critical, the heatspreading layer must be thermally connected to the areas of thesemiconductor portion 110 that will generate the most heat. Positioningthe heat spreading layer directly on the active side of the die 40 willbe effective because electronic devices on the active side will be indirect contact with the heat spreading layer 100. It would also bepossible to have intermediate structures or layers between thesemiconductor portion 110 and the heat spreading layer 100, provided theintermediate structures were thermally conducting.

[0043] The heat spreading layer 100, in one embodiment as shown in FIGS.3a-3 c is directly deposited on or within the die 40. There are noadditional layers between the heat spreading layer 100 and thesemiconductor portion 110. This configuration reduces the number ofinterfaces that heat must conduct through along it's path to ambient.Frequently, additional interfaces between materials will slow downconduction of heat. If additional layers were used between the heatspreading layer 100 and the semiconductor portion 110, additionalprocessing steps would also be required, which would increasemanufacturing process time.

[0044] As shown in FIGS. 3a-3 c, the die 40 with the heat spreadinglayer 100 of the invention, may be coupled to a Thermal InterfaceMaterial (TIM) 70, an external heat spreader 60, and a heat sink 80. Theaddition of the integral heat spreading layer serves to conduct heataway from local hot spots on the die 40 and distribute the heat acrossthe die 40. In this manner, the heat may be conducted via a largerhorizontal cross sectional area through the die 40 to the TIM 70, andeventually out to ambient.

[0045]FIG. 4 shows a magnified view of the die 40 and the heat spreadinglayer 100 from FIG. 3a. The semiconductor layer 110 may make up a bulksubstrate of the die 40, or it may only be a layer on top of anothersubstrate 120 such as glass. Electrical devices 150 are formed on thesemiconductor layer 110. It is understood that in context, formed “on”the semiconductor layer could mean on top of, within the semiconductorlayer, or partially within the semiconductor layer. It is alsounderstood that although not shown in the figures, the electricaldevices are electrically connected to each other by elements such asmetal traces. Some examples of electrical devices formed in anintegrated circuit die include but are not limited to transistors,diodes, and capacitors. Combinations of devices can be used to formhigher level circuits. Computationally intensive logic circuits are ofparticular interest due to their large heat generation. Floating PointUnit (FPU) circuits or Integer units can generate large amounts of heat,as well as other heat generating circuits of varying complexity.

[0046] The heat spreading layer 100 is formed to a thickness 102 overthe semiconductor layer 110, and over the electrical devices 150. Anelectrical connection 160 may be provided to transmit electrical signalsfrom the electrical devices 150 to the outside of the die 40 through theheat spreading layer 100. Although several thicknesses 102 of the heatspreading layer 100 are possible, a thickness of 0.05 mm or less ispractical, in order to accommodate the height of a typical electricalconnection 160. If the layer 100 is formed thinner than 0.05 mm, atradeoffs in heat spreading is a reduced vertical cross section ofmaterial in the layer 100 to conduct the heat with. An additionalconsideration in choosing the thickness of the layer 100 is the processtime necessary to form the layer 100. A thicker layer takes longer toform, and may become a process time limiting step. For this reason, itis advantageous to form a thin layer 100 using less process time.

[0047]FIG. 5 shows a die 40 with its active side up, with a heatspreading layer 100 covering a portion of the die surface. The heatspreading layer may cover the entire surface of the die 40 in oneembodiment. In another embodiment, the heat spreading layer may coveronly a select portion of the surface of the die 40 as show in FIG. 5.

[0048] The material selected for the heat spreading layer in theinvention must exhibit a thermal conductivity that is greater than thatof the semiconductor used in the die. A metal such as copper could beused with silicon as a semiconductor, because copper has a thermalconductivity around 3 times greater than silicon. In one embodiment ofthe invention, the heat spreading layer contains carbon. One type ofcarbon layer that is well suited to performing as a heat spreading layeris diamond. Diamond has a thermal conductivity around 20 times greaterthan silicon, and around 6 times greater than copper. Diamond is alsouseful because it is readily formed through processes such as chemicalvapor deposition (CVD). Although CVD is a process that is easilyincorporated into the die manufacturing process line, other depositionor attachment processes could be used and still be within the scope ofthe invention. Other possible carbon layer configurations could includecarbon 13, or variations of Buckminster fullerenes such as nanotubes.

[0049] The heat spreading layer 100 may be formed at any of severalperiods during the manufacturing process of the IC. One possible timefor formation of the heat spreading layer would be before any electronicdevices, such as transistors, were formed. Forming the heat spreadinglayer 100 first would allow a high temperature forming process for theheat spreading layer, without affecting any electronic devices, whichmay be affected by a high temperature process.

[0050]FIG. 6 illustrates a diamond crystal structure unit cell 200. Theunit cell 200 is a face centered cubic (FCC) structure made up of allcarbon atoms 210. The carbon atoms in the diamond unit cell 200 arebonded by SP3 bonds 220. In the practice of CVD diamond layerdeposition, the resulting layer commonly contains a large fraction ofdiamond, but is not entirely diamond. A typical characterization of adiamond layer is a percentage of SP3 bonding within the layer. A 100%pure diamond film would contain 100% SP3 bonds. The heat spreading layerin the invention as deposited by CVD does not require 100% SP3 bondingto be effective. However, a lower percentage of SP3 bonding results inless effective thermal conduction. In order to be effective, the heatconducting film must exhibit thermal conductivity that is greater thanthe semiconductor layer it is coupled to.

[0051]FIGS. 7a and 7 b show experimental data of a number ofcomputationally intensive logic areas on a semiconductor processor chip.A first location is at 10,S22, a second location is at 15,S22, and athird location is at 14,S15. While the results shown focus on aprocessor chip, other semiconductor chips could similarly benefit fromthe invention. FIG. 7a shows a thermal map of a semiconductor processorchip without an integral heat spreading layer. The three computationallyintensive logic areas show temperatures in the 100-110 degree Celsiusrange, and a very concentrated thermal gradient associated with theseareas. In contrast, FIG. 7b shows the same area of a semiconductorprocessor chip coated with a 0.05 mm layer of CVD diamond. The peaktemperature is in the 90-100 degrees Celsius range, and the thermalgradient associated with the three computationally intensive logic areasis more spread out as evidenced by the larger 80-90 degree Celsius mapregion.

CONCLUSION

[0052] A semiconductor chip has been shown that more effectivelytransmits heat from hot areas on the die to cooler regions on the die,and eventually to the thermal interface material as a result ofspreading the heat out over a larger cross sectional area. Local hotspots are minimized which allows the semiconductor chip to operate at ahigher frequency for a given upper threshold temperature.

[0053] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

We claim:
 1. A semiconductor chip, comprising: a semiconductor layer; anumber of electrical devices fabricated on the semiconductor layer; anda heat conducting layer integrally formed with the semiconductor chip,operatively connected to the semiconductor layer and adjacent to thenumber of transistors, the heat conducting layer having a higher thermalconductivity than the semiconductor layer.
 2. The semiconductor chip ofclaim 1, wherein the number of electrical devices includes a number oftransistors.
 3. The semiconductor chip of claim 1, wherein the heatconducting layer includes carbon.
 4. The semiconductor chip of claim 3,wherein the carbon includes diamond.
 5. The semiconductor chip of claim1, wherein the heat conducting layer is less than 0.05 mm thick.
 6. Thesemiconductor chip of claim 1, wherein the heat conducting layer is ontop of the semiconductor layer and the number of electrical devices. 7.The semiconductor chip of claim 1, wherein the semiconductor layer andthe number of electrical devices are on top of the heat conductinglayer.
 8. The semiconductor chip of claim 1, wherein the heat conductinglayer covers an entire surface of the semiconductor chip.
 9. Anelectronic assembly, comprising: a memory device; a semiconductorprocessor chip comprising: a semiconductor layer; a number of electricaldevices fabricated on the semiconductor layer; a heat conducting layerintegrally formed with the semiconductor layer and adjacent to thenumber of electrical devices, the heat conducting layer having a higherthermal conductivity than the semiconductor layer; and a system buscoupling the memory device to the semiconductor processor chip.
 10. Theelectronic assembly of claim 9, wherein the number of electrical devicesincludes a number of transistors.
 11. The electronic assembly of claim9, wherein the heat conducting layer includes carbon.
 12. The electronicassembly of claim 11, wherein the carbon includes diamond.
 13. Theelectronic assembly of claim 9, wherein the heat conducting layer isformed on top of the semiconductor layer and the number of electricdevices.
 14. The electronic assembly of claim 9, further including acooling device coupled to the heat conducting layer that transfers heatfrom the heat conducting layer to a location remote from thesemiconductor processor chip.
 15. A method of cooling a semiconductorchip, comprising: providing a number of electrical devices on asemiconductor layer of the semiconductor chip; integrally forming a heatconducting layer with the semiconductor layer, adjacent to the number ofelectrical devices, the heat conducting layer having a higher thermalconductivity than the semiconductor layer; conducting heat generated bythe number of electrical devices into the heat conducting layer; andtransmitting the heat generated by the number of electrical devicesthrough the heat conducting layer from a first region having a firsttemperature to a second region having a second temperature that is lowerthan the first region.
 16. The method of claim 15, wherein providing anumber of electrical devices includes providing a number of transistors.17. The method of claim 15, wherein coupling a heat conducting layer tothe semiconductor layer comprises coupling a carbon containing layer tothe semiconductor layer.
 18. The method of claim 16 wherein coupling acarbon containing layer to the semiconductor layer comprises coupling adiamond containing layer to the semiconductor layer.
 19. The method ofclaim 15, further comprising transmitting heat from the heat conductinglayer to a location remote from the semiconductor processor chip.
 20. Amethod of cooling a semiconductor chip formed from a semiconductingmaterial, comprising: integrally coupling a heat conducting layer to thesemiconductor chip, the heat conducting layer having a higher thermalconductivity than the semiconducting material; conducting heat from thesemiconductor chip into the heat conducting layer; and transmitting theheat through the heat conducting layer from a first region having afirst temperature to a second region having a second temperature that islower than the first region.
 21. The method of claim 20, whereincoupling a heat conducting layer to the semiconductor chip comprisescoupling a carbon containing layer to the semiconductor chip.
 22. Themethod of claim 21, wherein coupling a carbon containing layer to thesemiconductor chip comprises coupling a diamond containing layer to thesemiconductor chip.
 23. The method of claim 20, further comprisingtransmitting heat from the heat conducting layer to a location remotefrom the semiconductor chip.
 24. A method of manufacturing asemiconductor chip, comprising: fabricating a semiconductor layer;forming a number of electrical devices on the semiconductor layer;electrically connecting the number of electrical devices; and integrallyforming a heat conducting layer operatively connected to thesemiconductor layer, and adjacent to the number of electrical devices,the heat conducting layer having a higher thermal conductivity than thesemiconductor layer.
 25. The method of claim 24, wherein fabricating asemiconductor layer comprises fabricating a silicon substrate.
 26. Themethod of claim 24, wherein forming a heat conducting layer comprisesforming a carbon containing layer.
 27. The method of claim 26, whereinforming a carbon containing layer comprises forming a diamond containinglayer.
 28. The method of claim 27, wherein forming a diamond containinglayer comprises chemical vapor deposition (CVD) depositing a diamondlayer.